In 1964 Machin was chosen to design a new effigy of the Queen for the decimal coinage, which was to be introduced from 1968. This effigy was used for all British coins until 1984. It continued to be used on coins of New Zealand and Australia until 1985 and 1986 respectively, and on coins of Canada until 1989.

In 1966 the Queen approved Machin's similar design for an effigy of her to be used on what came to be known as the "Machin series" of British definitive postage stamps. Machin produced a bas-relief in clay, which when combined with a different coloured background, is reminiscent of the overlaid decoration of potteries such as Wedgwood. The design was first used on the 4d value which was issued in June 1967, and has been used on all British definitive stamps (except more recent regional issues) ever since. It is thought that this design is the most reproduced work of art in history with, to date, approximately 320 billion copies produced.[3]

In 2007 the Machin-designed stamp was still in use at its 40th anniversary and to mark the occasion, the Post Office issued a commemorative stamp featuring a photograph of Arnold Machin. It was also available for sale in a miniature sheet which incorporated another stamp with a reproduction of a Machin series stamp as well as two actual £1 Machins in different colours.

In the 1956, while resident at number 15, The Villas, Stokeville, (an estate of 24 Victorian houses in Stoke-upon-Trent) he received publicity in the national press when he chained himself to an old metal lamp-post in protest at its planned removal.[4] Machin's protest, "against the destruction of all the beautiful things which is going on in this country" did not prevent the lamp-post from being replaced by a concrete one; however, it was given to him for his own garden and his wife Patricia unlocked him. The lamp has since been restored to its original position.[5]

Machin and his wife Patricia had a son, Francis (1949-2007) who was an artist too and an architect. [6] After Francis died, the possessions of Arnold Machin in his house in rural Staffordshire were sold in auction on September and October 2008 by Cuttlestones.[7] The possessions included the fourth of the final plasters known made to create the Machin stamp series, the three others are kept in the Royal Mail archives.[8]